Pressurized water nuclear reactors have a vessel with the reactor core arranged therein and supported and maintained in the vessel by an assembly of components forming part of the lower internal equipment. Arranged above the core is the upper internal equipment which includes, in particular, the upper core plate and the guide tubes for the reactor control rods.
The installation of the upper internal equipment on the lower internal equipment must be carried out in an extremely precise manner, especially since it determines the proper operation of the reactor control rods. These control rods are introduced downwards into some core assemblies, by virtue of guiding members in the guide tubes of the upper internal equipment, which are aligned with the guide tubes for the fuel assemblies receiving the control rods.
The lower internal equipment comprises a cylindrical core-support shell with a vertical axis, with some of the upper internal equipment and especially the upper core plate engaged therein. At the time when the upper internal equipment is fitted onto the lower internal equipment, its centering must be ensured in a highly accurate manner and provision is made for four centering pins, integrally attached to the core support shell and arranged on its inner surface, at 90.degree. to each other, together with slots in the core support plate, at locations which correspond to those of the centering pins. The entry of the centering pins into the slots at the time when the upper internal equipment is installed on the lower internal equipment enables it to be centered.
When a nuclear reactor is being built, the operations of installing the upper internal equipment on the lower internal equipment are carried out in a workshop, and this operation, which requires a great deal of care, does not, however, present special difficulties, since it is relatively easy to gain access to all the parts of the internal equipment which are being adapted and fitted.
For the repair and maintenance of nuclear reactors which have already operated it has been proposed to change completely the upper internal equipment, in the event where this equipment may have been subjected to wear or distortions in service. The irradiated lower internal equipment is then kept in the vessel and new upper internal equipment is adapted onto the irradiated internal equipment remaining in the vessel. For this purpose, the vessel is opened and filled with water, and the various checks which enable the new upper internal equipment to be adapted onto the irradiated lower internal equipment must be carried out from a distance, for example from the platform associated with the devices for assembling the upper internal equipment, above the upper part of the nuclear reractor vessel opening out into the pool.
To facilitate the adaptation of the new upper internal equipment to the irradiated lower internal equipment which may have undergone a slight distortion, the clearance between the slots in the upper core plate and the corresponding centering pins is increased. This increased initial clearance enables the upper internal equipment to be introduced without the risk of jamming, and then to be installed while the perfect alignment of the guide tubes is monitored.
The relative position of the slots and of the centering pins must then be accurately known, in order to machine with very great accuracy the adapters which are fixed onto the upper core plate in the region of the slots and which enable the clearance between the slots and the centering pins to be returned to the required value.
So far there has been no known device permitting the measurement of clearances around the pins for centering the upper internal equipment of a pressurized water nuclear reactor with very high accuracy, from a distance and under water.
The object of the invention is therefore to offer a device for measuring clearance around a pin for centering the upper internal equipment of a pressurized water nuclear reactor on the lower internal equipment placed in the reactor vessel which is open and full of water, the lower internal equipment comprising the reactor core enclosure on which the centering pins are fixed, and the upper internal equipment comprising the upper core plate in which slots which correspond to the centering pins are machined, this device being capable of permitting an extremely accurate measurement of clearance between the centering pins and the slots in the upper core plate, when the upper internal equipment is installed on the lower internal equipment remaining in the vessel, especially in the case where the irradiated lower internal equipment requires the measurements to be carried out from a distance.
To this end, the measuring device according to the invention comprises:
(a) a carrier integrally attached to a pedestal equipped with means intended to cooperate with holes in the upper core plate for fixing the carrier onto the upper core plate;
(b) at least two travellers mounted in the carrier and parallel to the bearing surface of the pedestal which is placed on the upper face of the upper core plate;
(c) two movable slides mounted on the two travellers, each carrying a planar marking plate parallel to the direction of travel of the slides and each equipped with a sensing finger projecting downwards relative to the bearing face of the pedestal;
(d) resilient means, mounted inside the carrier, for returning the slides to bring them back towards each other in the direction of the travellers; and
(e) a marking assembly carried by the carrier and comprising two styli mounted in guidance carriers directed at right angles to the marking plates, each placed opposite a marking plate and pushed by a spring towards these plates, and a control equipment fixed integrally to the styli and to a control cable, whose end away from the control equipment enables a pull to be applied to separate the styli from the plates, while each of the return springs is compressed, while the styli mark the plates when the cable is released.